Oxy-fuel combustion process

ABSTRACT

Production of oxygen-enriched gas streams is disclosed herein. Air streams contact an oxygen-selective mixed conductor particularly a perovskite material whereby oxygen is retained or adsorbed on the perovskite and can be employed in a variety of processes such as in combusting a fuel gas, heat recovery and boiler related operations.

[0001] This application claims priority from Provisional U.S. Patent Applications 60/346,582 filed Jan. 8, 2002; 60/346,597 filed Jan. 8, 2002; and 60/347,268 filed Jan. 10, 2002.

BACKGROUND OF THE INVENTION

[0002] The primary purpose of combustion processes is to generate heat. In a power plant or in an industrial boiler system, the heat is utilized to generate high pressure steam which in turn may be used to provide process heating or may be used to produce electricity. Most conventional combustion processes utilize air as a source of oxygen. The presence of nitrogen in air does not benefit the combustion process and may even create problems. For example, nitrogen will react with oxygen at combustion temperatures forming nitrogen oxides (NOx), an undesirable pollutant. In many cases, the products of combustion must be treated to reduce nitrogen oxide emissions below environmentally acceptable limits. Moreover, the presence of nitrogen increases the flue gas volume which in turn increases the heat losses and decreases the thermal efficiency of the combustion process. Additionally, high nitrogen content in the flue gas may make it unattractive to capture CO₂ either as a product or for sequestration. With the current emphasis on CO₂ sequestration to alleviate harmful effects of global warming, it is critical to develop processes which will enable CO₂ capture in a cost effective way.

[0003] One way to eliminate nitrogen from the combustion exhaust or flue gas is to use pure oxygen in the combustion process instead of air. However, combustion with oxygen generates very high temperatures and therefore some of the flue gas produced must be recycled to moderate temperatures. This in turn dilutes the oxygen content to about 27% (remaining ˜73% is CO₂ and water) and maintains the flame temperature to the same value. While such a scheme would eliminate the problems associated with nitrogen, the cost of oxygen at present is too high to make it economically attractive.

[0004] Production of oxygen-enriched gas stream using ion transport ceramic membrane is discussed in U.S. Pat. No. 5,888,272 which discloses a process for separating a feed gas stream into an oxygen-enriched gas stream which is used in a combustor and an oxygen-depleted gas stream. The feed gas stream is compressed, and oxygen is separated from the compressed feed gas stream using an ion transport module including an ion transport membrane having a retentate side and a permeate side. The permeate side of the ion transport membrane is purged with at least a portion of a combustion product gas stream obtained from the combustion in the combustor of the gas stream exiting the permeate side of the ion transport module. The disadvantages of this method of oxygen production are the high cost of fabrication of the membrane and the difficulty in producing membrane structures that are leak-proof. Also, oxygen recovery is typically low in membrane units.

[0005] The present invention is based on the use of high-temperature, oxygen-selective ceramic materials made in particulate form to produce a substantially nitrogen-free oxygen stream suitable for oxy-fuel application, and may provide an attractive option to reduce oxygen cost. Such systems utilize either pressure swing or temperature swing mode since the oxygen retention capacity of the ceramic material is strongly dependent on temperature and pressure. The process normally operates at temperatures greater than 300° C. and offers several advantages, including high oxygen capacity and large oxygen selectivity. A key advantage of this process is that it uses the oxygen-selective material in conventional pellet form in fixed bed reactors, which can be designed using traditional methods. Thus, the process can be commercially adopted more easily compared to the membrane based process mentioned above, which requires special fabrication, sealing and assembly procedures, and is known to have several issues in this regard. An additional advantage of the fixed bed, ceramic-based system is that it can directly produce an oxygen containing stream, substantially free of nitrogen, with the oxygen concentration suitable for oxy-fuel application. This is unlike conventional processes, such as cryogenic air separation method, which first produce high purity oxygen, and require subsequent dilution to get the required oxygen concentration.

[0006] The present invention is aimed at reducing the cost of oxygen by producing substantially nitrogen-free oxygen containing stream suitable for combustion processes. It relates to the use of a high-temperature, oxygen generation system to produce an oxygen-containing stream, substantially free of nitrogen. More particularly, it describes the use of an oxygen-selective ceramic material to separate oxygen from an air stream to produce an oxygen containing stream which can be employed in an industrial boiler or fired heater or in other combustion based processes as an oxygen source instead of air.

SUMMARY OF THE INVENTION

[0007] The present invention provides for a method for producing an oxygen stream for use in an industrial boiler or fired heater. A process is described wherein a part of the flue gas from the boiler, primarily containing water vapor and CO₂, is used to sweep a reactor containing oxygen-saturated high temperature oxygen-selective ceramic material (e.g., perovskite) to produce an oxygen-containing stream. The oxygen-containing stream is fed to the boiler along with a fuel, which is burned in the boiler to generate heat. The oxygen-depleted ceramic material is saturated with oxygen by exposing it to air in a cyclic fashion. Therefore, the process for operating the ceramic system consists of at least two steps in each cycle of the cyclic operation. In the first step, an air stream is introduced into the reactor containing the high temperature oxygen-selective ceramic material, which selectively retains oxygen. In the second step, a portion of the flue gas from the boiler is fed into the reactor to purge out at least a part of the oxygen from the ceramic material, so that the material becomes oxygen-depleted. The oxygen retention step is exothermic while the oxygen removal step is endothermic. The overall process is thermo-neutral, in principle; however, some heat loss will occur, which needs to be compensated.

[0008] In one embodiment of the process, the boiler is operated under slightly under-oxidized conditions, so that the flue gas contains no oxygen, but contains a small amount of CO and H₂. The CO+H₂ is burned in the reactor with a portion of the oxygen retained in the ceramic material to generate heat required to sustain the cyclic operation of the reactor.

[0009] In another embodiment of the process, the boiler is operated under conditions such that the fuel is completely burned, and a small amount of excess 02 is present in the flue gas (typically ˜0.5-5.0 vol. %). In this case, the recycled flue gas is fed to the reactor along with the addition of a small amount of suitable fuel (CO, H₂, CH₄, etc. or a combination thereof), in the amount at least sufficient to react with the excess oxygen present in the flue gas. The combustion catalyst may be combined with the oxygen-selective ceramic material in the same reactor, as a layer at the entrance. Also, a layer of perovskite can act as a combustion catalyst. This combustion generates heat necessary for the cyclic process. The amount of fuel gas added is adjusted so as to generate sufficient heat. Any excess fuel added reacts with the oxygen stored in the ceramic material. If higher temperature results due to the combustion, it helps extract more oxygen from the ceramic material since the amount of oxygen retained in the ceramic material generally decreases with increasing temperature.

[0010] Optionally the flue gas can be passed through an additional reactor to which a controlled amount of fuel gas is added. The reactor may contain a catalyst, such as a supported noble metal catalyst. The oxygen is consumed in this reactor by reaction with the added fuel. As described above, a portion of the resulting gas, after heat recovery, is then fed to the reactor for generating the oxygen-containing gas stream.

[0011] If high temperature valves are used, the hot flue gas from the boiler can be fed directly into the reactor. When low temperature valves are used, the hot flue gas from the boiler is first passed through a heat exchanger to recover the heat and to generate steam as a useful product, before it is fed to the reactor. The portion of the flue gas, which is not recycled may be used to capture CO2 from it after separating water and other impurities.

[0012] In another embodiment of the process, the oxygen-containing gas leaving the reactor is cooled to separate the water in the stream by condensation, thereby increasing the concentration of oxygen in the stream returning to the boiler. The increased oxygen concentration may provide more flexibility in the operation of the boiler.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a schematic representation of a boiler and the ceramic oxygen generation system as practiced by the present invention.

[0014]FIG. 2 is a schematic representation of the ceramic oxygen generation system for oxyfuel application as practiced in the present invention.

[0015]FIG. 3 is a schematic representation of a ceramic oxygen generation system with steam purge as practiced in the present invention.

[0016]FIG. 4 is a schematic representation of ceramic oxygen generation reactor showing the layer arrangement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017]FIG. 1 is a schematic embodiment of a boiler or fired heater and an oxygen generation ceramic system B. Contained therein in B is the oxygen-selective ceramic material. Line 10 carries fuel gas to the boiler A. The fuel can be selected from the group consisting of CH₄, H₂, CO, C₂H₄, C₂H₆ and mixtures thereof or can be coal, char or other solids as well as various refinery waste streams, fuel oils, etc. or any suitable combustible material. The combustion exhaust gas or flue gas, which consists primarily of carbon dioxide and water vapor, exits combustion/heat recovery zone A through line 12. A part of the combustion exhaust gas is directed through line 14 to the oxygen generation system B. Compressed air enters the oxygen generation system through line 20. Oxygen lean stream containing mainly nitrogen, up to 98%, exits the oxygen generation system through line 22. Oxygen from the air is retained onto the oxygen-selective ceramic material. The combustion exhaust gas enters the system B, removes this oxygen and regenerates the ceramic material. The gas leaves through line 18 as substantially nitrogen-free oxygen rich gas and enters the boiler A whereby combustion can occur anew.

[0018] The ceramic system primarily comprises at least 2 reactors filled with high temperature oxygen-selective ceramic material, such as perovskite material, and an inert ceramic material for internal heat exchange, optional multi-pass heat exchangers and switchover valves. The process is cyclic and may be compared to a pressure swing retention process. Briefly, air is passed into first bed where oxygen is preferentially retained onto the material and oxygen lean stream is withdrawn from the top of the bed. Once the material becomes at least in part saturated with oxygen, the operation is transferred to another vessel. The first bed is now purged with the combustion exhaust gas or recycled flue gas, which removes at least part of the oxygen and as a result also regenerates the material. Minimum two reactors are required to ensure continuous operation.

[0019] Turning now to FIG. 2, air is compressed, and, after passing through multi-pass heat exchanger, will pass through one of the beds, which contains high temperature oxygen-selective ceramic material, such as perovskite material. Oxygen will be retained on the perovskite and nitrogen will leave the bed as effluent. This effluent gas stream will then pass up again through one of the multi-pass heat exchangers and will leave the cyclic system. While one bed is undergoing the air step, the second perovskite bed which is already partially saturated with oxygen is purged with the recycled flue gas stream. Like air, the recycled flue gas also passes through a multi-pass heat exchanger before passing through the perovskite bed. As the recycled flue gas passes through the bed, it picks up the oxygen stored on the perovskite and also regenerates the perovskite. The oxygen rich gas then leaves the bed through the multi-pass exchanger, exchanging heat with the incoming recycled flue gas.

[0020]FIG. 2 is described here with bed B on air step and bed A on recycled flue gas or regeneration step. Air is first compressed to the desired pressure using air blower E. The compressed air is fed to the multi-pass heat exchanger G through valve V5. Valve V6 is closed during this step. Air is heated in exchanger G by exchanging heat with the returning oxygen-lean stream 16. The heated air, 14, is fed to the perovskite bed B. The oxygen-lean stream, 15, exits bed B, exchanges heat with incoming air in exchanger G and then leaves the system through valve V8 as stream 20.

[0021] Recycled flue gas from the boiler system is first cooled in cooler C and then compressed in blower D prior to feeding it into multi-pass heat exchanger F through valve V1. Once heated, it passes through bed A, which is saturated with oxygen. The oxygen rich stream, 35, leaves the bed from the bottom, passes through the exchanger F and into the buffer tank H through valve V3.

[0022] A typical valve sequence is given in the table below: Duration Bed A Bed B Valves Step Sec Feed Feed V1 V2 V3 V4 V5 V6 V7 V8 1 30 Air Flue open close open Close open Close open Close Gas 2 30 Flue Air close open close Open close Open close Open Gas

[0023] The present invention can be integrated with a boiler or fired heater in several ways with an objective of improving the efficiency. In one embodiment of this process, the boiler is operated under slightly under-oxidized conditions so that the flue gas contains no oxygen but contains a small amount of carbon monoxide and hydrogen. The carbon monoxide and hydrogen are burned in the perovskite reactor to generate heat required to sustain and improve the cyclical operation of the perovskite reactor.

[0024] Alternatively, the boiler is operated under conditions such that the fuel is completely burned and a small amount of excess oxygen is present in the flue gas, typically about 0.5 volume %. In this case, the recycle flue gas is fed to the perovskite reactor along with the addition of a small amount of a suitable fuel such as carbon monoxide, hydrogen, methane or a combination thereof in an amount at least sufficient to react with the excess oxygen present in the flue gas (stream 50 in FIG. 2). This combustion generates heat necessary for the cyclical process. The amount of fuel gas added is adjusted so as to generate sufficient heat. Any excess fuel added reacts with the oxygen stored on the perovskite. If higher temperature results due to the combustion, it helps extract more oxygen from the perovskite.

[0025] Alternatively yet, the boiler is operated under conditions of excess oxygen to assure complete combustion of all the fuel. In this case, the flue gas can contain up to 5% by volume oxygen. This flue gas is passed through an optional reactor to which a controlled amount of fuel gas as described above is added. The reactor may contain a catalyst such as a supported noble metal catalyst. The oxygen is consumed in this reactor by a reaction with the added fuel gas. As described above, a portion of the resulting gas after heat recovery is then fed to the perovskite reactor for generating the oxygen-containing gas stream. The combustion catalyst can be separate or may be combined with the perovskite in the same reactor, as a layer at the entrance to the reactor. Also, a layer of perovskite can act as a combustion catalyst.

[0026] Alternatively, the oxygen-containing gas leaving the perovskite reactor is cooled to separate the water in the stream as condensate thereby increasing the concentration of oxygen in the stream returning to the boiler. The increased oxygen concentration may be beneficial to the boiler operation and may provide more flexibility to the operation of the boiler. An extension of this scheme is to use steam only as a regeneration gas as shown in FIG. 3. The main advantage of this scheme is that oxygen can be produced in any concentration by cooling the oxygen-rich stream and condensing the steam out. Since the process still operates at low pressure, only low-pressure steam is necessary. The availability of low-pressure steam is usually not a problem as schemes presented here are integrated as part of an overall boiler or power plant.

[0027] In one embodiment, water is removed from the recycled flue gas before it enters the ceramic oxygen generation system so that it consists of mainly CO₂. It has been discovered that when the purge gas in the oxygen extraction step is CO₂, the amount of oxygen recovered from the ceramic bed is higher compared to other gases such as N₂ or steam. This is believed to be due to exothermic retention of CO₂ on the ceramic material leading to greater oxygen release.

[0028] The schemes presented in FIGS. 2 and 3 are based on partial pressure swing process i.e. the driving force for extraction of stored oxygen is provided by the difference in partial pressure of oxygen between the oxygen retention and extraction steps. The pressure to which the air is compressed is mainly determined by the required concentration of oxygen in the oxygen-rich stream. According to the invention, air is fed at a pressure of 15-400 psia, preferably 15-100 psia, and more preferably 20-40 psia, and the recycled flue gas at 0.1-200 psia, preferably 8-50 psia, and more preferably 10-30 psia, so that the pressure difference between the two streams at the entrance to the reactor is maintained between 5 and 20 psi.

[0029] The schemes presented here relate to the concepts employed in ensuring efficient heat management. For example, one aspect of the invention provides for the use of inert materials for regenerative heat transfer in cyclic catalytic processes. The reactor configuration with inert materials is shown in FIG. 4. In particular, such regenerative heat transfer is used in conjunction with at least one external heat exchanger to achieve the desired heat transfer for the overall process. Through heat exchange with these inert materials, temperatures of hot gas streams exiting a reactor can be significantly reduced, e.g., to below about 900° C., and preferably as low as about 500° C. Such a reduced gas stream temperature allows use of low-cost construction materials, and results in corresponding cost reduction, as well as an increased operating life of the external heat exchanger required for additional heat transfer.

[0030] While such a heat transfer scheme is generally applicable to any cyclic process, it is particularly well-suited for processes with relatively high operating temperatures, e.g., about 250° C. or higher, where the unavailability of switchover valves for high temperature operation necessitates that all hot gas streams be effectively cooled so that standard valves can be employed. Furthermore, it is also well-suited to cyclic processes with relatively short cycle times, such as those in which the heating and cooling times are below about a minute, e.g., between about 15 to about 60 seconds.

[0031] According to embodiments of the invention, multi-pass compact heat exchangers are used to carry out supplemental heat transfer from hot gas streams. These include two external heat exchangers, which operate on cyclic duty in synchronization with the cyclic operation of the reactors. The heat exchange is further complemented with the internal regenerative heat exchange using inert layers of ceramic material. The external heat exchangers allow heat exchange between the inlet and outlet of the same streams, for example air and waste nitrogen stream or recycled flue gas and oxygen-rich streams. On the other hand, internal regenerative heat exchange allows heat exchange between two different streams, for example air and oxygen rich stream and waste nitrogen and recycled flue gas. This heat exchange philosophy also allows the use of low temperature switchover valves and enhances the reliability of the cyclic process.

[0032] The multi-pass exchangers, which are a part of the compact heat exchanger family, offer significant thermal advantages over conventional shell and tube exchangers. They are available commercially and may be employed for pressures as high as 2000 bar and temperatures as high as 800° C. A detailed review of compact heat exchangers can be found in an article by V. V. Wadekar, in CEP, December 2000, which is herein incorporated by reference. For high temperature applications, these heat exchangers are typically fabricated from stainless steel or other alloys.

[0033] While multi-pass exchangers are integral part of the schemes presented here, it may also be possible to adjust process parameters to complete all heat exchange using inert materials placed inside the reactor. This will eliminate the need for external heat exchange. On the other hand, it is also possible to carry out all heat exchange in heat exchangers thereby eliminating the need for inert layers within the reactor vessels.

[0034] One characteristic of cyclic processes is the possibility of contamination of the desired product stream with impurities as a result of vessel voids. For the present case, this means that the oxygen rich stream may get contaminated with nitrogen present in the voids at the end of the oxygen retention step. In order to avoid this, an additional step may be introduced. In this step the reactor will be rinsed with steam after the oxygen retention step. This will remove any nitrogen that may be present in the voids. The reactor now can be purged with the combustion exhaust gas or flue gas.

[0035] The oxygen-selective ceramic materials are typically oxygen-selective mixed conductors, which exhibit both high electronic and oxygen ionic conductivities at elevated temperature. Examples of these mixed conductors are perovskite-type oxides, CeO₂-based oxides, Bi₂O₃-based oxides, ZrO₂-based oxides, and brownmillerite oxides. In order to further enhance its electronic conductivity and catalytic activity for oxygen ionization, some metal phase can be added into the ceramic material to form a ceramic-metal composite. The metals can be selected from Cu, Ni, Fe, Pt, Pd, Rh and Ag.

[0036] In general, the oxygen-selective ceramic materials retain oxygen through conduction of oxygen ions and filling up the oxygen vacancies in its bulk phase. The oxygen retention capacity usually increases with increasing oxygen partial pressure and decreasing temperature. Therefore, the retention and release of oxygen into and from the ceramic material during retention and release steps perform efficiently in that the oxygen partial pressure during the retention step is much higher than that in the release step.

[0037] In a preferred embodiment, the at least one oxygen-selective ceramic material comprises an oxygen-selective mixed ionic and electronic conductor. In a more preferred embodiment, the oxygen-selective ceramic material comprises a perovskite-type ceramic having the structural formula A_(1−x)M_(x)BO_(3−δ), where A is an ion of a metal of Groups 3a and 3b of the periodic table of elements or mixtures thereof; M is an ion of a metal of Groups 1a and 2a of the periodic table or mixtures thereof; B is an ion of a d-block transition metal of the periodic table or mixtures thereof; x varies from >0 to 1; and δ is the deviation from stoichiometric composition resulting from the substitution of ions of metals of M for ions of metals of A.

[0038] In a more preferred embodiment, the at least one oxygen-selective ceramic material is a perovskite-type ceramic and x varies from about 0.1 to 1.

[0039] In another more preferred embodiment, the at least one oxygen-selective ceramic material is a perovskite-type ceramic and A is one or more f-block lanthanides. In a more preferred embodiment, A is La, Y, Sm or mixtures thereof.

[0040] In another more preferred embodiment, the at least one oxygen-selective ceramic material is a perovskite-type ceramic and M is at least one metal of Group 2a of the periodic table of elements. In a more preferred embodiment M is Sr, Ca, Ba or mixtures thereof.

[0041] In another more preferred embodiment, the at least one oxygen-selective ceramic material is a perovskite-type ceramic and B is Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn or mixtures thereof. In a more preferred embodiment, B is V, Fe, Ni, Cu or mixtures thereof.

[0042] In another more preferred embodiment, the at least one oxygen-selective ceramic material is a perovskite-type ceramic and x is about 0.2 to 1.

[0043] In another more preferred embodiment, the at least one oxygen-selective ceramic material is a perovskite-type ceramic and A is La, Y, Sm or mixtures thereof, M is Sr, Ca or mixtures thereof, and B is V, Fe, Ni, Cu or mixtures thereof.

[0044] In another embodiment, the at least one oxygen-selective ceramic material conductor is a member selected from the group consisting of (1) ceramic substances selected from the group consisting of Bi₂O₃, ZrO₂, CeO₂, ThO₂, HfO₂ and mixtures thereof, the ceramic substances being doped with CaO, rare earth metal oxides or mixtures of these; (2) brownmillerite oxides; and (3) mixtures of these.

[0045] In another embodiment, the at least one oxygen-selective ceramic material conductor is at least one ceramic substance selected from the group consisting of Bi₂O₃, ZrO₂, CeO₂, ThO₂, HfO₂ and mixtures of these, and the at least one ceramic substance is doped with a rare earth metal oxide selected from the group consisting of Y₂O₃, Nb₂O₃, Sm₂O₃, Gd₂O₃ and mixtures of these.

EXAMPLES Example 1

[0046] Preparation of La_(0.2)Sr_(0.8)Co_(0.6)Fe_(0.4)O_(3−δ) Perovskite Powder

[0047] The powder of perovskite-type oxide was prepared first by mixing of corresponding metal oxides or hydroxides and then repeated steps of sintering, ball-milling and filtration for three times. The temperatures in 3 sintering steps were, respectively, 900° C., 950° C. and 1000° C., and the sintering time was 8 hours. The first sintering was conducted right after dry-mixing of La₂O₃, Sr(OH)₂.8H₂O, Ni₂O₃, Co₂O₃ and Fe₂O₃. The ball the material was carried out with grinding media and water after each sintering. The solid was collected by filtration after ball milling. The filtration cake was dried at 100° C. for overnight before it was subjected to the next sintering. After the last ball-milling, the dried filtration cake was crushed and ground into fine powder. The final powder had a perovskite-type phase structure.

Example 2

[0048] Fabrication of La_(0.2)Sr_(0.8)Co_(0.6)Fe_(0.4)O_(3−δ) perovskite extrudates

[0049] The perovskite-type oxide powder made in Example 1 was transformed into a slurry after addition of about 5 wt % hydroxyethyl cellulose and 14.5 wt % water. Thus obtained slurry was aged overnight before it was loaded into an extruder and transformed into extrudates (3 mm in diameter and 4 mm in length). The extrudates were dried in an oven at 90° C. for about 2 hr, and then calcined at 600° C. for 5 hr. The extrudates were finally sintered at 1050° C. for 8 h. The final extrudates were porous and mechanically strong.

Example 3

[0050] The extrudates made in Example 2 were packed in a tubular reactor made of high temperature metal alloy. The reactor was designed in such a way that the gas streams of air, CO₂ and steam could be fed into the reactor from either the top end or the bottom end of the reactor as required. Mass flow controllers controlled the flow rates of the gas streams. The reactor temperature and valves were controlled with PLC. The product and waste streams during purge and retention steps were collected in a tank, and their average compositions were analyzed with a gas analyzer and a GC. In the experiment, the reactor temperature was controlled at 825° C. An air stream at 7.6 slpm and a CO₂ stream at 4.7 slpm were alternately fed into the reactor for every 30 seconds in a counter-current fashion. The reactor pressures were kept at 23.7 psia and 18.7 psia respectively during air and CO₂ steps. During the last 2 seconds of the air step, the reactor pressure decreased from 23.7 psia to 18.7 psia. The average product composition during CO₂ step was: 27.8% O₂, 67.1% CO2 and 7.4% N2, while the waste stream generated during air step contained 2.3% O2, 12.5% CO2 and 83.5% N2. This demonstrates that an oxygen-rich stream containing primarily CO2 and O2 can be produced with the process disclosed in this invention

Example 4

[0051] In this experiment, an air stream at 7.6 slpm and a stream of CO₂+steam mixture at 4.5 slpm were alternately fed into the reactor described in Example 3 for every 30 seconds in a counter-current fashion. The reactor pressures were kept at 23.7 psia and 18.7 psia respectively during air and CO₂+steam steps. The average product composition (on a dry basis) during CO₂+steam step was: 40.8% O₂, 44.5% CO2 and 14.7% N2, while the waste stream generated during air step contained 3.7% O₂, 11.4% CO2 and 84.9% N2. This result indicates that an oxygen-rich stream can be produced with a mixture of CO2 and steam as purge gas using the process disclosed in this invention.

Example 5

[0052] In this experiment, an air stream at 7.6 slpm and a stream of steam at 6.2 slpm were alternately fed into the reactor described in Example 3 for every 30 seconds in a counter-current fashion. The reactor pressures were kept at 23.7 psia and 18.7 psia respectively during air and steam steps. The average product composition (on a dry basis) during steam step was: 70.4% O₂, 29.6% N2, while the waste stream generated during air step contained 0.3% O2 and 99.7% N2 (with trace amount other non-oxygen gases). This result showed that an oxygen-rich stream can be produced with stream as purge gas using the process disclosed in this invention. TABLE 1 Summary of the results in Examples 3-5 Example Product Waste Stream # Flow O₂ % CO₂ % H₂O % N₂ % Flow O₂ % CO₂ % N₂ % 3 Dry 5.36 27.8 67.1 0 7.4 7.02 2.3 12.5 83.4 wet 5.36 27.8 67.1 0 7.4 7.02 2.3 12.5 83.4 4 Dry 3.82 40.8 44.5 0 14.7 5.75 3.7 11.4 84.9 wet 6.06 25.7 28.1 37.0 9.3 5.75 3.7 11.4 84.9 5 Dry 3.37 70.3 0 0 29.6 6.99 0.3 0 99.7 wet 9.59 24.7 0 64.9 10.4 6.99 0.3 0 99.7

[0053] Table 1 summarizes the results in Examples 3-5 and compares the product compositions on the wet basis, i.e. including the steam in the product stream. As shown, O₂ concentration in the product on the wet basis increases with increasing CO₂ concentration in the purge gas, indicating that CO₂ has stronger regeneration capability than steam. As noted in the examples, there was some amount of nitrogen still presented in the product stream due to the void space in the reactor. This nitrogen can be easily eliminated from the void space by an additional step between the air and the purge gas steps.

[0054] While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention. 

Having thus described the invention, what we claim is:
 1. A method of combusting a fuel gas in a combustion zone comprising the steps of: (a) feeding into said combustion zone said fuel gas; (b) feeding into said combustion zone an oxygen-enriched gas from an oxygen retention system; (c) combusting said fuel gas; and (d) recovering and recycling the combustion exhaust gas from said combustion zone to said oxygen retention system.
 2. The method as claimed in claim 1 wherein said oxygen retention system contains a ceramic adsorbent.
 3. The method as claimed in claim 2 wherein said ceramic adsorbent is an oxygen-selective mixed conductor.
 4. The method as claimed in claim 3 wherein said oxygen-selective mixed conductor is a perovskite type ceramic having the structural formula A_(1−x)M_(x)BO_(3−δ).
 5. The method as claimed in claim 4 wherein A is a rare earth ion, M is Sr, Ca, Ba, V or mixtures of these; B is Co, Mn, Cr, Fe or mixtures of these; x varies from greater than 0 to about 1; and δ is the deviation from stoichiometric composition resulting from the substitution of Sr, Ca and Ba for rare earth ions.
 6. The process as claimed in claim 5 wherein x varies from about 0.1 to about
 1. 7. The process as claimed in claim 4 wherein A is La, Y or mixtures of these; M is Sr, Ca or mixtures of these; and B is Co, Fe or mixtures of these.
 8. The process as claimed in claim 4 wherein x is about 0.2 to
 1. 9. The method as claimed in claim 1 wherein said oxygen-enriched gas is delivered at temperatures greater than 150° C.
 10. The method as claimed in claim 1 wherein said oxygen-enriched gas is produced at pressures of about 1 to about 20 bar.
 11. The method as claimed in claim 1 wherein said oxygen retention system produces oxygen-enriched gas through a two-step process of retention and purge.
 12. The method as claimed in claim 11 wherein oxygen is adsorbed from an oxygen-containing feed gas stream.
 13. The method as claimed in claim 12 wherein nitrogen is removed from said retention system
 14. The method as claimed in claim 1 wherein high purity nitrogen is produced as a by-product during the oxygen retention step.
 15. The method as claimed in claim 1 wherein said oxygen retention system comprises two or more adsorbent beds.
 16. A method for producing oxygen-enriched gas for use in a combustion zone comprising the steps: (a) feeding air to a retention system; (b) retaining oxygen from said air onto an oxygen-selective mixed conductor; (c) removing nitrogen from said retention system; (d) feeding oxygen-enriched gas to said combustion zone; (e) combusting a fuel gas in the presence of said oxygen-enriched gas; and (f) feeding the exhaust gas from said combustion zone to said retention system.
 17. The method as claimed in claim 16 wherein said method is cyclical.
 18. The method as claimed in claim 16 wherein a portion of said exhaust gas from step (f) is withdrawn.
 19. The method as claimed in claim 18 wherein CO₂ is recovered from said exhaust gas.
 20. The method as claimed in claim 16 wherein said oxygen retention system contains a ceramic adsorbent.
 21. The method as claimed in claim 20 wherein said ceramic adsorbent is an oxygen-selective mixed conductor.
 22. The method as claimed in claim 21 wherein said oxygen-selective mixed conductor is a perovskite type ceramic having the structural formula A_(1−x)M_(x)BO_(3−δ).
 23. The method as claimed in claim 22 wherein A is a rare earth ion, M is Sr, Ca, Ba, V or mixtures of these; B is Co, Mn, Cr, Fe or mixtures of these; x varies from greater than 0 to about 1; and δ is the deviation from stoichiometric composition resulting from the substitution of Sr, Ca and Ba for rare earth ions.
 24. The process as claimed in claim 23 wherein x varies from about 0.1 to about
 1. 25. The process as claimed in claim 24 wherein A is La, Y or mixtures of these; M is Sr, Ca or mixtures of these; and B is Co, Fe or mixtures of these.
 26. The process as claimed in claim 25 wherein x is about 0.2 to
 1. 27. The method as claimed in claim 16 wherein said oxygen-enriched gas is produced at temperatures greater than 300° C.
 28. The method as claimed in claim 16 wherein said oxygen-enriched gas is produced at pressures of about 1 to about 20 bar.
 29. The method as claimed in claim 16 wherein said oxygen retention system produces oxygen-enriched gas through a two-step process of retention and purge.
 30. The method as claimed in claim 29 wherein oxygen is adsorbed from an oxygen-containing feed gas stream.
 31. The method as claimed in claim 30 wherein nitrogen is purged from said retention system.
 32. The method as claimed in claim 31 wherein said oxygen retention system comprises two or more adsorbent beds.
 33. A method for combusting a gas stream and recovering heat from said combustion comprising the steps: (a) passing an air gas stream into a retention system containing an oxygen-conducting ceramic; (b) retaining oxygen from said air gas stream onto said oxygen-conducting ceramic; (c) passing a combustible gas over said oxygen-conducting ceramic whereby said combustible gas combusts in the presence of the retained oxygen producing carbon dioxide, H₂O and heat; and (d) recovering said carbon dioxide, H₂O and heat in the form of super-heated steam.
 34. The method as claimed in claim 33 wherein said retention system is a circulating fluidized bed reactor.
 35. The method as claimed in claim 33 wherein a fuel stream is passed over said oxygen-conductive ceramic in step (c).
 36. The method as claimed in claim 33 wherein said fuel stream comprises CH₄, H₂, CO, C₂H₄, C₂H₆ and mixtures thereof.
 37. The method as claimed in claim 33 wherein said ceramic adsorbent is an oxygen-selective mixed conductor.
 38. The method as claimed in claim 34 wherein said oxygen-selective mixed conductor is a perovskite type ceramic having the structural formula A_(1−x)M_(x)BO_(3−δ).
 39. The method as claimed in claim 38 wherein A is a rare earth ion, M is Sr, Ca, Ba, V or mixtures of these; B is Co, Mn, Cr, Fe or mixtures of these; x varies from greater than 0 to about 1; and δ is the deviation from stoichiometric composition resulting from the substitution of Sr, Ca and Ba for rare earth ions.
 40. The process as claimed in claim 39 wherein x varies from about 0.1 to about
 1. 41. The process as claimed in claim 40 wherein A is La, Y or mixtures of these; M is Sr, Ca or mixtures of these; and B is Co, Fe or mixtures of these.
 42. The process as claimed in claim 40 wherein x is about 0.2 to
 1. 43. A method of operating a boiler to generate heat comprising the steps: (a) passing air over an oxygen-conducting perovskite in a reactor system and retaining oxygen on said oxygen-conducting perovskite; (b) passing the effluent gas from said boiler to said oxygen-conducting perovskite; and (c) feeding a gas stream containing oxygen to said boiler with a fuel gas wherein said gas stream combusts in said boiler to fuel said boiler.
 44. The process as claimed in claim 43 wherein said process is cyclic.
 45. The method as claimed in claim 43 wherein said ceramic adsorbent is an oxygen-selective mixed conductor.
 46. The method as claimed in claim 44 wherein said oxygen-selective mixed conductor is a perovskite type ceramic having the structural formula A_(1−x)M_(x)BO_(3−δ).
 47. The method as claimed in claim 46 wherein A is a rare earth ion, M is Sr, Ca, Ba, V or mixtures of these; B is Co, Mn, Cr, Fe or mixtures of these; x varies from greater than 0 to about 1; and δ is the deviation from stoichiometric composition resulting from the substitution of Sr, Ca and Ba for rare earth ions.
 48. The process as claimed in claim 47 wherein x varies from about 0.1 to about
 1. 49. The process as claimed in claim 48 wherein A is La, Y or mixtures of these; M is Sr, Ca or mixtures of these; and B is Co, Fe or mixtures of these.
 50. The process as claimed in claim 48 wherein x is about 0.2 to 1.51. A method of converting a feed gas to a product gas in a cyclical process comprising the steps: (a) introducing said feed gas containing an oxidant into a first reactor, wherein said first reactor contains a catalyst contained between inert materials having heat transfer properties disposed at each end of said first reactor, and said first reactor having an opening at both ends wherein at least one heat exchanger with channels is connected to said openings of said first reactor; wherein said first feed gas is preheated by heat transfer with said product gas in said heat exchanger prior to introducing said first feed gas into said first reactor; (b) withdrawing a first product gas from said first reactor; (c) introducing a second flow of said feed gas into a second reactor, wherein said second reactor contains a catalyst contained between inert materials disposed at each end of said second reactor and said second reactor having an opening at both ends wherein at least one heat exchanger with channels is connected to said openings of said second reactor; wherein said second feed gas is preheated by heat transfer with said product gas in said heat exchanger prior to introducing said second feed gas into said second reactor; (d) withdrawing a second product gas from said second reactor; (e) diverting said first feed gas flow into said second reactor thereby forming said first product gas and diverting said second feed gas flow into said first reactor thereby forming said second product gas.
 52. The method as claimed in claim 51 wherein said feed gas is a reducing gas.
 53. The method as claimed in clam 52 wherein said reducing gas is natural gas.
 54. The method as claimed in claim 51 wherein said first product gas and said second product gas are the same gas.
 55. The method as claimed in claim 51 wherein said product gas is a mixture of carbon monoxide and hydrogen.
 56. The method as claimed in claim 51 wherein said catalyst is a perovskite type mixed conductor. 